Photon Mass and Momentum: Train Experiment

In summary: The light bulb will emit light even though it is not being directly exposed to the sun. This is because the light bulb has been in a state of 'conservation of energy'. According to some, this means that the light bulb has existed even though it was not being directly observed.
  • #1
lntz
54
0
Hello,

I have a question that i'd like to ask, but the wording is a little tricky, so I hope you can get the gist of what I'm trying to describe.

I was thinking about the thought experiment that is often used to describe relative motion - a light clock on a train passing through a station being observed from the platform. From there I had an idea that isn't directly related, but it came out of the train.

Imagine you have a train traveling along a straight piece of track and on board there is a photon emitter aiming perpendicular to the direction of travel. (This is where I don't think I explain it too well). Before the photons have been emitted, can they be said to exist? I ask this because I'm wondering whether they already have momentum in direction the train is traveling in when they are emitted. So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion?

I hope that makes sense and that this is in the correct part of the forum,

Thanks in advance,

Lntz.
 
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  • #2
lntz said:
So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion?
According to train observers, the path of the photon is perpendicular to the direction of the train's travel. But according to platform observers, the path is at an angle.
 
  • #3
Thanks for the reply.

I should have thought this one through! I really should have realized the path would be different for the two observers...
 
  • #4
lntz said:
I really should have realized the path would be different for the two observers...
Right. The speed of light is invariant, but not the velocity (direction).
 
  • #5
Before the photons have been emitted, can they be said to exist?

That depends on the nature of your source, not on it's relative motion. Suppose you have a light bulb 'on' in a closed box and you transmit by opening a door in the box. Then suppose you keep the light turned 'off' until you wish to transmit.
 

1. What is a photon?

A photon is a fundamental particle that carries electromagnetic energy. It has zero rest mass and travels at the speed of light.

2. How is the mass of a photon measured?

The mass of a photon is typically measured indirectly through its energy and wavelength using the equation E=mc². The mass of a photon can also be calculated using the Planck's constant and the speed of light.

3. What is the relationship between mass and momentum in a photon?

In a photon, the momentum is directly proportional to its energy and inversely proportional to its wavelength. This means that as the energy of a photon increases, its momentum also increases, while its wavelength decreases.

4. How does the "Train Experiment" demonstrate the relationship between photon mass and momentum?

The "Train Experiment" involves two identical train cars, one stationary and one moving at a constant velocity. A light beam is directed from the front of the moving train car to the back, and the experiment shows that the photons have the same momentum and energy before and after the beam is emitted, despite the train's motion. This demonstrates the conservation of momentum and the relationship between mass and momentum in a photon.

5. Can a photon have mass and momentum at rest?

No, a photon cannot have mass or momentum at rest. Since a photon travels at the speed of light, it has no rest mass and its momentum is always related to its energy and wavelength.

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